The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
Since the discovery of carbon nanotubes (CNTs), numerous efforts have been focused on finding technological approaches to lower their corresponding synthesis temperature, which could make the growth of these nano-materials more practical for various applications especially in the nano-electronic and field emission areas [1-4]. More specifically, since complementary metal-oxide-semiconductor (CMOS) devices are generally fabricated at temperatures below 400° C., lowering the CNT growth temperature is imperative for their incorporation in hybrid, complex electronic devices [5]. However, the most common growth temperatures for CNTs, depending on the catalyst system and the synthesis conditions, range between 700-1000° C. [6-9]. Compared to other synthesis techniques, chemical vapor deposition (CVD) remains the most popular method and has been widely used to synthesize CNTs at lower temperatures in controlled conditions resulting in nanomaterials with excellent morphologies and characteristics [10]. More recently, several groups have been working to synthesize CNTs at very low temperatures (below 300° C.) with and without the presence of catalyst systems. Wang et al. have reported the growth of multi-walled carbon nanotubes (MWCNTs) at 160° C. by the decomposition of polyethylene glycol using a hydrothermal synthesis without the addition of catalyst Fe/Co/Ni [11]. Since this reaction requires 20 hours, it is time-consuming and inconvenient. Wang et al. were able to improve the quality and yield of MWCNTs by increasing the synthesis temperature to 180° C. [12]. Vohs et al. reported the lowest synthesis temperature (175° C.) for MWCNTs, using CCl4 as a precursor along with metal-encapsulated dendrimers as catalysts in a 24-hour treatment at 27.6 MPa [13]. However, this method was reported to produce relatively low-quality nanotubes and could have limited practicality for industrial applications, since it requires high pressure and a lengthy synthesis time.
Moreover, low-quality MWCNTs or nanorods were synthesized on various catalysts, such as Fe, Au, or Ag, utilizing tetrachloroethylene as the carbon feedstock; reactions were performed in the presence of benzene and potassium and kept at 200° C. for 27 hours [14]. Vertically aligned MWCNTs have been synthesized using a photo-thermal chemical vapor deposition technique on a Ti/Fe catalyst film at temperatures as low as 370° C. [5]. Others have demonstrated the growth of CNTs using the CVD method on catalyst systems, such as Ni supported on zeolite and Ti/Fe on SiO2 from methane at temperatures between 350-600° C. [15]. Additionally, Sharma et al. have reported in situ observations of nanotube growth over Ni or Co catalysts at 450° C. [10].
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.